# rec.photo.darkroom

## Subject: Re: Calibrated Thermometers

Well - the experiment continues. I remembered that I have a cheap digital
thermometer (used for measuring snow temps). This is the self-contained pen
style that go for about \$20. It has more than sufficient range for both the
freezing point and boiling point of water. Here's a link:
http://www.professionalequipment.com/xq/ASP/ProductID.3915/id.22/subID.174/qx/default.htm
The stated accuracy is +/- 1.8 degrees F over the range of 32 - 212 F, which
would not be sufficient, in my opoinion to insure phot chemicals were at the
right temperature. So I decided to at least test the end points to see just
how accurate those were.

I got out some distilled water ice cubes (used to mix working developer on
ultra warm days) and used the blender to make a slush of ice and steam
distilled water. After stirring for a few minutes, the thermometer
stabilized at 32.1 degrees F. I found that if I didn't stir the temp was
about 32.3 - 32.5, and I theorize that the shaft of the probe may have been
conducting enough room heat south to create a "microclimate" around the
probe. Stirring would keep the colder water around it all the time in such
a case.

I next dipped it in a pan of slowly boiling water (but not touching the
bottom). The thermometer stabilized at 212. Since it does not read
decimals when displaying higher F temps, I tried changing to the C scale and

So my cheezy digital thermometer isn't too far off at those two points. If
I can assume good liniarity in the range between them, I should be able to
use it to calibrate my dial darkroom thermometers and my water temp control
valve. The amount of error at the two "end points" of the calbration are
well within my limits - I'm just developing B&W, so a 1/10 of a degree plus
or minus isn't the end of the world. Heck, it's almost impossible for me
(even with a controlled water temp valve) to hold any chemical to a 1/10
degree tolerance when in use over time anyway.

As suggested elsewhere, perhaps I'll try for a "mid range" value to
determine if there the cheezy digital thermometer has a linear scale. In
the meantime, thanks for all the useful discussion. Hope nobody got a
bloody nose!

"Floyd L. Davidson" wrote in message
news:87ac9q798c.fld@apaflo.com...
> "Nicholas O. Lindan" wrote:
>>"Pieter Litchfield" wrote
>>
>>> If I put the thermometer in a glass of water full of ice cubes, should
>>> it
>>> not read 32 degees F (or 0 degrees C)?
>>
>>No.
>>Try it.
>
> Yes. And you really should try it.
>
>>http://www.its-90.com/wtpguide.html
>>
>>The short form is:
>>
>> o The ice is melting in the water so the water must be
>> above freezing.
>>
>>Ergo, the water is > 32F
>
> Or exactly equal to 32F.
>
>> o The ice that has not melted is below freezing. If
>> cold enough it will freeze the water. For water to freeze
>> it must be supercooled.
>>
>>Ergo, the water is < 32F
>
> Or exactly equal to 32F.
>
>>At some time in the process the water may pass through 32F.
>
> Yep, as soon as all of the ice is melted.
>
>>When demonstrated by the 7th grade science teacher cold ice
>>(<32F) is added to the mix as the teacher stirs and peers at
>>the thermometer. When enough ice has been added for the
>>thermometer to pass through 32F the teacher promptly announces
>>success and tosses the mixture in the sink. If the teacher
>>had kept going and the the water was pure enough and well
>>polished with no air in it and the ice cold enough the water
>>can get as cold as -40F/C.
>
> Have you ever seen water at -40 that was not ice?
>
> Or better yet, have you ever even so much as seen -40?
>
>>> I can't use boiling water. Are there any other useful points? I would
>>> think that if there is a way of setting the calibration near 68 degrees
>>> F
>>> to 75 degreesF
>>
>>To do this you need a known good thermometer to know that the
>>water bath is at 68F so the questionable thermometer can be read
>>to determine it's error.
>
> Glad you got something right!
>
> --
> Floyd L. Davidson